Outline 1 Introduction 2 Basic IC fabrication processes 3 Fabrication techniques for MEMS 4 Applications 5 Mechanics issues on MEMS
2.2 Lithography Reading: Runyan Chap. 5, or 莊達人 Chap. 7, or Wolf and Tauber Chap12~14 Lithography is the process to define the desired pattern on thin films or substrates substrate lithography
Semiconductor sensor edited by S.M. Sze, 1994
Basic steps of lithography Initial status Substrate Photoresist Mask fabrication Photoresist coating Light source Exposure Photoresist with desire pattern Final status (after development)
2.2.1 Photoresist (PR) The PR is a photosensitive organic material which contains three ingredients + resin - solvable in aqueous developer + sensitizer - photosensitive but insolvable in aqueous developer + solvent - keep the resist in liquid state until it is coated to the substrate The major functions of PR are + Precise pattern transformation + Protection of the thin film (or substrate) underneath during etch
Positive and negative resist The following discussions are mainly focused on the positive photoresist Exposure Positive photoresist Negative photoresist
Step 1 Dehydration baking Basic Steps in PR Processing Dehydration baking is to remove the moisture absorbed by the substrate from the atmosphere. Therefore the adhesion can be improved. Step 2 Coating photoresist The PR is spin coated on the substrate surface. The thickness of the PR is determined by the spin rate and the viscosity of the PR. In order to improve adhesion, hexamethyldisilazane (HMDS) is also spin coated onto the substrate before the PR. Substrate PR Substrate holder
Step 3 Soft bake The substrate is baked near 90 C to drive solvent out of the PR (from 20 ~ 30% to 4 ~ 7%) so that it can be properly exposed Step 4 Exposure The PR is exposed to a light source (usually Ultraviolet light) to define its pattern Step 5 Development o After the PR is exposed, the regions without protecting by the mask will be removed by developer Step 6 Hard bake The substrate is baked at 90~120 C to (1) Improve PR adhesion (2) Increase the etch resistance of the PR (3) Remove the solvent remain in the PR to prevent solvent-burst effect in the vacuum environment * The PR will be very difficult to be removed if its post bake temperature is too high
2.2.2 Mask Fabrication Mask - a mask is defined as a pattern tool which contains patterns that can be transferred to an entire wafer in one exposure (Wolf and Tauber, 1986) Mask for single exposure process W. Fang, Ph.D. Thesis, 1995
Mask for multiple exposure process - alignment marks are necessary structure patterned by mask1 Mask 1 structure patterned by mask2 Mask 2
Misalignment of a surface micromachined suspension misaligned suspension alignment key W. Fang and J.A. Wickert, DSSC annual report, Carnegie Mellon University, 1993
Photo reduction technique Drawing the pattern Cut the desired pattern on rubylith Make the reticle by photography + The film of the camera is the mask + The size of the pattern is reduced after taking pitchure on the rubylith Light source Rubylith Optical system (Camera) Glass film (Reticle)
The working mask is made by a step-repeat equipment (stepper) + Place the first mask into the stepper + The stepper will shrink and duplicate the pattern on the reticle Reticle Optical system (Camera) Light source Glass film (Working mask) Distortion may occur for each process Moved by controller
Optical pattern generation technique The mask fabricated through this technique is a glass (or quartz) plate with chromium (Cr) pattern on its surface The pattern is generated, stored, and transferred to a mask making machine by computer The desired pattern is decomposed into rectangles before transferred to the mask making machine quartz plate PR Cr Exposure and development Etch and remove PR Wolf and Tauber, Silicon Processing for the VLSI Era Vol.1, 1986
The dimension of rectangle is defined by the shutter. In addition, the shutter is able to give a desired angle for the rectangle by rotating The mask making machine The rectangles are positioned by a motorized X-Y stage The throughput of this technique depends on the complexity of the desired pattern The technique is applied to make reticle Semiconductor sensor edited by S.M. Sze, 1994
2.2.3 Exposure Contact exposure + The earliest method + The mask contacts with the substrate during exposure + This method gives the most reliable transferred image contact + The mask will be worn or contaminated after frequently contacting with the substrate Proximity exposure + The mask and substrate is close to each other but not contact proximity + The resolution is decreased as the separation is increased
Projection exposure + The mask and substrate is not contact + The resolution is outstanding + Multiple exposure is required to expose the whole substrate Wolf and Tauber, Silicon Processing for the VLSI Era Vol.1, 1986
Important issues + Resolution : In general, the term resolution describes the ability of an optical system to distinguish closely spaced objects Hecht, Optics, 1987
Here we refer to the minimum resolution of a lithography machine as the minimum line width or space that the machine can precisely print The minimum line width: 1. For contact and proximity approaches: L = λ m g distance between mask and substrate 2. For projection approach: λ Lm = = NA λ n sinθ θ numerical aperature index of refraction Lens
Optical sources
+ Contrast Resist contrast is a measure of the sharpness of the transition from exposure to non-exposure non-exposure E i : the amount of energy required for the positive resist to just begin to break down (ideal case) slope = log E f 1 log E i E f : the minimum energy required for the positive resist to be completely removed E i E f fully exposure Runyan and Bean, Semiconductor Integrated Circuit Processing Technology, 1990.
+ The actual dimensions and slope of an exposed and developed resist E f Wolf and Tauber, Silicon Processing for the VLSI Era Vol.1, 1986 + Adhesion : If the adhesion of thin film and photoresist is poor, the thin film protected by the photoresist will be attacked in the etching process
Wolf and Tauber, Silicon Processing for the VLSI Era Vol.1, 1986
2.2.4 Advanced Lithography Techniques X-ray lithography + Short wavelength, high resolution + High throughput + Very difficult to make an X-ray mask thick metal layer (10~20μm) Thin layer (~2μm) Holder Electron Beam (E-beam) lithography + Short wavelength, high resolution + Direct writing the pattern on resist without mask is possible + Very low throughput + Expensive
Electron Beam Technique In this technique an electron beam is applied to write a pattern onto the resist of a mask The mask fabricated through this technique is also a glass (or quartz) plate with chromium (Cr) pattern on its surface, however the PR is replaced by the E-beam resist W. Menz, Microsystem Technology for Engineerings Intensive Course, 1994.
The pattern is constructed by plenty of stripes which are defined by two aperatures No double exposed problem The throughput of this technique depends on the complexity of the desired pattern W. Menz, Microsystem Technology for Engineerings Intensive Course, 1994.
Conclusions The current lithography technique already satisfied the requirement for MEMS Compare with deposition, lithography is more straightforward, since there is less option The challenge of MEMS lithography : highly structured surface